1. Field of the Invention
This invention relates to reducing heat energy in semiconductors and, more particularly, to thermal conductors embedded within the semiconductor for removing heat therefrom.
2. Description of the Related Art
Modern VLSI chips require cooling to improve reliability of their circuits and interconnects, to increase their circuit switching performance, and to regulate thermally generated noise in their circuits. Cooling reduces the likelihood that a metal wire will form voids or a contact will become open. It also reduces the extent of time-dependent transistor mobility and threshold degradation which adversely affects circuit performance and operation. Furthermore, in typical CMOS microprocessors, every reduction in temperature of 10 degrees Celsius produces a 2% rise in operating frequency. For CMOS transistors, high temperatures yield significantly larger leakage currents, due to the thermal generation of carriers. This deleterious current doubles every 11 degrees Celsius and is known to adversely affect the functional operation of dynamic and analog circuits.
As field effect transistor (FET) channel length decreases, the leakage current grows exponentially. Leakage current will become a dominant source of circuit noise unless circuit operating temperatures can be decreased through cooling. Ideally, chips should be cooled close to absolute zero.
In reality, cooling systems are constrained by economic and technical considerations. Present cooling systems vary widely in complexity and cost. Personal computers use small fans to remove hot air from circuit boards whereas high cost mainframes use plumbing to circulate liquid coolant to every chip. Engineering capital has been expended to remove heat from a collection of chips attached to a circuit board or thermal conduction module.
Cooling may be applied selectively to individual chips within a board. One such example is described in U.S. Pat. No. 4,935,864 to W. L. Schmidt et al.(Schmidt). Schmidt describes a means to preferentially cool individual chips on a circuit board with a thermoelectric chiller. As circuit integration density grows exponentially, individual hot spots within a chip will require an on-chip means to dissipate heat.
Motivation for integrating heat reduction into a semiconductor chip becomes obvious from a thermal map of the current generation of high speed memory chips. Referring to FIG. 1, cross hatched regions 12 on a memory chip 10 approximate regions where hot spots may occur. High temperature areas correspond to the physical locations of a bit decode, a word decode, and sense amplifier circuitry which switch often while performing memory chip functions. Little heat is generated in memory cell quadrants 14 because memory cells are composed of small transistors and are used infrequently. Temperature differences occur wherever circuit diversity exists, and such diversity exists in every VLSI chip from memory to microprocessors.
It would be advantageous for integrated circuit technology with ever increasing power density to distribute heat more evenly throughout a chip. In U.S. Pat. No. 5,621,616 to A. H. Owens (Owens), a heat dissipation technique is provided. Owens describes a high-conductivity heat transfer pathway which draws heat away from the semiconductor substrate, through the various metal levels and vias, through a solder bump. The heat is drawn into the chip carrier where heat may be removed by convection to the ambient air. Owen proposes to embed metal plugs deep into the chip substrate to collect the heat generated by the transistors and then to remove that heat through metal interconnect already existing in VLSI chips.
Very Large Scale Integrated (VLSI) circuits shrink to improve performance and thereby increase operating frequency. Heat is generated due to constant switching of these devices. The removal of this heat becomes a major obstacle to the efficient performance of theses devices. Therefore, a need exists for a heat conductor for semiconductors. The material of the heat conductor should provide high thermal conductivity and low electrically conductivity. It is therefore advantageous to integrate diamond thermal conductors within a semiconductor chip to distribute heat more evenly throughout the chip, conduct heat away from hot areas in a chip, and conduct heat to the chip's exterior.